Paper No.
11060
2011
CRUDE OIL CHEMISTRY EFFECTS ON INHIBITION
OF CORROSION AND PHASE WETTING
François Ayello1, Winston Robbins, Sonja Richter and Srdjan Nešić
Institute for Corrosion and Multiphase Technology
Department of Chemical and Biomolecular Engineering
Ohio University
342 West State Street,
Athens, OH 45701
ABSTRACT
The effect of crude oil chemistry on corrosion processes is poorly understood. A simple
mechanistic model that takes into account the combined effects of corrosion inhibition and
wettability alteration, due to the adsorption of polar molecules onto the steel surface, is proposed.
While the inhibition effect is dominant in oil-water stratified flow, the wettability alteration is
dominant in turbulent flow. Consequently, both effects have to be taken into consideration in
order to predict corrosion in oil-water pipe flow. The objective of this study is to identify chemicals
present in crude oil which have an effect on inhibition and wettability. The likeliest molecules to
adsorb onto a metal surface are polar compounds. Nine polar compounds containing nitrogen,
sulfur and oxygen have been studied. Corrosion rates were recorded, the change of the steel
wettability has been quantified and the mass adsorbed onto the metal surface measured with a
quartz-crystal-microbalance. Results show that certain groups of chemicals have strongly
inhibitive effect on corrosion and wettability while others had only a negligible effect.
Keywords: adsorption, corrosion, crude oil, inhibition, mercaptans, naphthenic acid, polar
compound, pyridinic, pyrrolic, sulfide, thiophenes, wettability
INTRODUCTION
The objective of this study is to determine which groups of chemicals present in crude oil have an
effect on crude oil inhibition of corrosion and phase wetting. This information is necessary in order
to predict corrosion rates in pipelines where crude oil and water are flowing in a mild steel pipe.
However, the molecular composition of crude oil is very complex and crude oil chemistry varies
from field to field. It is therefore impossible to study the effect of each and every chemical present
in crude oil.
1
Current affiliation: Det Norske Veritas, 5777 Frantz Road, Dublin, OH, USA 43017
©2011 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE
1 in this paper are
International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed
solely those of the author(s) and are not necessarily endorsed by the Association.
1
The main phenomenon studied here was the adsorption of particular surface active compounds
naturally present in crude oil: aromatics as well as oxygen, sulfur, and nitrogen containing
compounds. Only a few compounds were chosen for this study each representative of a particular
chemical class. The adsorption of surface active compounds at the metal surface can create an
organic protective film leading to corrosion inhibition1,2,3. The presence of an organic film at the
metal surface in oil-water two-phase flow can also change the steel/water and steel/oil interfacial
tension, thus changing the wettability of the steel. Wettability has a direct effect on the corrosion
rate4, i.e. steel wet by oil does not corrode.
Another phenomenon studied was the accumulation of surface active compounds at the oil-water
interface. This accumulation of surface active compounds changes the oil-water interfacial
tension5,6 which influences the break-up process of the water phase by the oil phase7.
Surface active compounds adsorption effects on inhibition of corrosion and wettability have been
studied both qualitatively and quantitatively in the past8-13. However, these effects were studied
only by using corrosion measurements. The use of one single experiment cannot distinguish
which effect has the largest influence on corrosion. Therefore, in this study each effect will be
investigated separately without the interference of the others. The main aim is to achieve a better
understanding of corrosion in oil-water two-phase flow.
Hypothesis: Crude Oil Chemistry Has Three Effects on Corrosion
Surface active compounds are known for their affinity towards iron surfaces, where they can
adsorb either via weak interactions such as van der Waals forces (characteristic of physisorption)
or strong covalent bonding (characteristic of chemisorption)14,15. Adsorption processes can create
a very thin organic film on metal surfaces. This accumulation of surface active compounds at the
metal surface, where the oxidation of iron by protons occurs, can slow down corrosion rates.
Furthermore, the accumulation of the surface hydrophobic active compounds can change the
wettability of iron, which is hydrophilic by nature16,17.
Another hypothesized effect of surface active compounds in crude oil-water two-phase flow is in
the change of the minimum velocity needed in order to disperse the water phase in droplets4 due
to a change of the oil-water interfacial tension.
The current study is thus based on the three following hypotheses:
1 - Corrosion inhibition is induced by the accumulation of surface active compounds at the metal
surface.
2 - Wettability of steel is altered by accumulation of surface active compounds at the metal
surface.
3 - Flow pattern is modified by accumulation of surface active compounds at the oil-water
interface.
Surface Active Compounds Tested
As explained earlier, it is impossible to study the effect of each and every compound present in
crude oil on inhibition of corrosion and phase wetting. However, based on the three hypotheses
above, only surface active compounds have the potential to adsorb onto an iron surface. There
are four dominant classes of surface active compounds in crude oil: aromatics, oxygen containing
compounds, sulfur containing compounds and nitrogen containing compounds. For each class
the subclasses will be identified and tested (Table 1).
Aromatic compounds – adsorb onto the steel surface by sharing -electron density from the
aromatic ring with the metal surface15. This binding can possibly decrease corrosion rates or
2
2
change the steel-oil interfacial tension. The aromatic chosen for this study is 1,2,3,4tetrahydronaphthalene.
Oxygen containing compounds – adsorb onto the steel surface by the interaction of electron
density, as manifested by unshared electron pairs on the oxygen within the molecule, with the
metallic surface18. The resulting physical interaction blocks the metal’s active site and therefore
decreases the corrosion rate19. The class of oxygen containing compounds with the highest
potential for adsorption in crude oil is organic acids. Organic acids adsorb onto the steel surfaces
via the electrons on the carbonyl oxygen. While small chain organic acids are known to increase
corrosion rates20, long chain organic acids are used as corrosion inhibitors in industry21,22. Acetic
acid will be used as representative of small molecule organic acids and myristic acid will
represent long chain organic acids. Moreover, to validate the results found with the myristic acid,
a commercially available mixture of naphthenic acids extracted from kerosene will be tested. The
molecular formula used for the naphthenic acids tested is R-CH2-COOH where R represents a
mixture of 0 to 3 ring saturated 5- and 6-membered ring structures. Average molecular weight for
the naphthenic acids tested is 214 g·mol-1. Mass spectrometry suggests that two ring structures
are prevalent.
Sulfur containing compounds – these can adsorb in an analogous fashion to oxygen containing
compounds, using unshared electrons, although they have a greater tendency to chemisorb via
formation of sulfur-metal bonds. Sulfur containing compounds are subdivided in two categories:
non-polar (sulfide, thiophenes) and polar forms (thiols commonly known as mercaptans)23. Dioctyl
sulfide, dibenzothiophene and tetradecanethiol were tested to represent sulfide, thiophenes and
mercaptans, respectively.
Nitrogen containing compounds – these compounds can adsorb in an analogous fashion to both
oxygen and sulfur containing compounds. Nitrogen containing compounds are subdivided in two
categories: pyridinic forms (known as basic nitrogen) and pyrrolic forms (known as neutral
nitrogen)23. Acridine was chosen to represent pyridinic compounds and carbazole to represent
pyrrolic compounds.
Table 1: Description of the surface active compounds used in the present research.
Chemical
class
Aromatic
Oxygen
containing
compounds
Name
Formula
1,2,3,4Tetrahydronaphthalene
C10H12
132.2
Acetic Acid
CH3COOH
60.05
Myristic acid
CH3(CH2)12COOH
228.37
Commercially available
naphthenic acid mixture
Dibenzothiophene
Sulfur
containing
compounds
Molecular
weight
-1
g·mol
R(CH2)COOH
~214
184.26
C12H8S
Dioctyl sulfide
CH3(CH2)7S(CH2)7CH3
258.51
1-Tetradecanethiol
CH3(CH2)13SH
230.45
3
3
Nirogen
containing
compounds
Carbazole
C12H9N
167.2
Acridine
C13H9N
179.2
Steel Wettability
Hydrocarbons are hydrophobic. Their adsorption at the metal surface can change the affinity of
the steel, which is hydrophilic in nature, to hydrophobic. This alteration of phase wetting at the
metal surface is caused by a change of steel-oil and steel-water interfacial tension8. The
difference between the steel-oil interfacial tension and the steel water interfacial tension is termed
the equilibrium spreading coefficient,  12 and is determined by Young’s equation (Eq.1).
Oil
Water
s/w
o/w
s/o
Steel
Figure 1: Interfacial tension forces applied to a water droplet in model oil resting on a steel
surface. The shape of the droplet is determined by the interaction of the interfacial
forces of oil-water (o/w), steel-oil (s/o) and steel-water (s/w).
Young’s equation:
   o / w cos   s / o   s / w
If the contact angle is larger than 90° the equilibrium spreading coefficient
(1)

is positive.
Therefore, the affinity of the steel for water (s/w) is stronger than the affinity of the steel for oil
(s/o). On the other hand, if the affinity for the steel is stronger for oil, the water droplet will have a
contact angle smaller than 90°. A strong affinity of the steel for oil is beneficial for preventing
corrosion. In this condition, a water droplet approaching the wall surface will not wet the metal
and therefore will not be corrosive.
Interfacial Tension: Effect of the Flow on Corrosion
The wetting of the internal surface of the pipe depends on what happens both at the steel surface
and in the bulk (flow effect). If the turbulence of the oil phase is strong enough to break the water
phase into droplets and entrain them in the flow, the water droplets theoretically never touch the
pipe wall. This condition can be predicted by a set of four equations called the water wetting
model (Eq.2), which was proposed by Nesic et.al4. When the condition described by (Eq.2) is
4
4
satisfied, the model predicts that the water phase flows as droplets in a continuous oil phase. The
pipe is therefore free from corrosion problems.

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
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

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c
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

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
 min 
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0 .6
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 w   o   cos    g
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 o   o

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(2)
The prediction of the model depends on hydrodynamic parameters such as oil velocity, water
velocity, water cut, density of the oil, inclination and size of the pipe and also on the oil-water
interfacial tension (). The crude oil chemistry does not have a direct effect on hydrodynamic
parameters but it can change the oil-water interfacial tension. Polar compounds can be present
which have a hydrophilic head attached to a hydrophobic tail. The head can be in the aqueous
phase while the tail resides in the oil phase. The accumulation of such polar compounds at the
oil-water interface decreases the oil-water interfacial tension26.
The oil-water interfacial tension is a key parameter in the breaking process of the water phase
into entrained droplets by the turbulence of the oil phase. Strong oil-water interfacial tension
makes it more difficult for the turbulence to break the water phase into droplets. Therefore, it is
easier to obtain a stratified flow, which can be corrosive since water is then in direct contact with
the pipe wall.
EXPERIMENTAL PROCEDURE
Each hypothesis will be tested by a specific experimental technique. The first hypothesis, that
surface active compounds that accumulate at the metal surface induce corrosion inhibition, is
tested by corrosion measurements. The second hypothesis, that surface active compounds that
accumulate at the metal surface change the wettability of the steel, is tested by contact angle
measurements. The third hypothesis, that surface active compounds at the oil-water interface
have an effect on the flow pattern, is tested by interfacial tension oil-water measurements. All
tests were performed at room temperature and atmospheric pressure.
Corrosion Measurements
A 2 Liter glass cell apparatus mounted with a rotating electrode was used for the corrosion
measurements. The working electrode (rotating cylindrical electrode, diameter 1.5 cm) is made
from carbon steel. The composition of the steel is shown in Table 2. The counter electrode was a
platinum ring and the reference electrode a silver/silver-chloride electrode connected to the
solution by a salt bridge. Linear polarization resistance (LPR), electrochemical impedance
spectroscopy (EIS) and potentiodynamic sweep data were recorded.
Table 2: Steel composition in wt.% (Fe is in balance).
Al
0.004
Ni
0.16
As
0.010
P
0.015
B
0.0002
Pb
0.017
C
0.19
S
0.013
Ca
0.002
Sb
0.015
Co
0.007
Si
0.22
Cr
0.13
Sn
0.021
Cu
0.16
Ta
<0.001
Mn
0.83
Ti
<0.001
Mo
0.042
V
0.058
Nb
0.003
Zr
0.002
A specific experimental procedure was designed to understand the adsorption of a surface active
compound present in the oil phase onto the metal surface. In a 2L glass cell, 1.8L of aqueous
electrolyte was introduced (1 wt. % NaCl, 1 bar CO2, pH 5.0). The working electrode was
sequentially polished with 400 and 600 grit sandpaper then cleaned with acetone and isopropanol
in an ultrasonic bath. The working electrode was then rotated (2000 rpm) in the glass cell. The
5
5
corrosion rate was measured using LPR, after which 200 mL of model oil (LVT-200) was mixed
with a particular surface active compound and introduced in the glass cell. The rotating working
electrode was moved up into the oil phase for 20 minutes, and then moved down into the water
phase. The corrosion rate was recorded every 5 minutes by LPR. Excellent agreement was found
between LPR, EIS and polarization sweep data, therefore only LPR measurements will be
presented in this paper.
Steel Wettability Measurements
The wettability test procedure was as follows. One percent by weight of surface active compound
was added to 1 L of model oil. The oil phase (model oil, 800mL) and water phase (1% NaCl, pH5,
1bar CO2, 200mL) are mixed together in a beaker for 1 hour, then left to settle overnight to
equilibrate. A flat carbon steel coupon was cleaned with acetone and polished sequentially with
400 and 600 grit sandpaper, then further cleaned with isopropanol in an ultrasonic bath for 2
minutes. The coupon was then dried and immersed in the model oil. A droplet of water was then
added on the top of the coupon. A video recording of the evolution of the droplet at the metal
surface was taken over 10 minutes, allowing for the measurement of the contact angle water
droplet-on the steel as a function of time.
Interfacial Tension Measurements
To determine the effect of flow on corrosion, interfacial oil-water tension was measured with a
platinum ring tensiometer. The preparation of the solution was the same as outlined above for the
contact angle measurements. One percent by weight of surface active compound was added to 1
L of model oil. The oil phase (model oil, 800mL) and water phase (1% NaCl, pH5, 1bar CO2,
200mL) were mixed together in a beaker for 1 hour and then left to settle overnight to equilibrate
prior to testing.
Adsorption Measurements
Hypothesis 1 and 2 are based on an adsorption mechanism, i.e., the adsorption of surface active
compounds onto iron. This adsorption can be recorded as a function of time by a Quartz Crystal
Microbalance (QCM). The quartz crystal is covered with an iron layer. In each experiment, a new
iron-coated quartz crystals was introduced into a 1L beaker containing 0.5L of model oil. After the
quartz crystal frequency was stabilized, 0.5L of model oil mixed with one surface active
compound was added into the beaker. The frequency of the quartz crystal was recorded
throughout the whole experiment. The observed change in frequency is proportional to a change
of mass. Therefore, the adsorption of surface active compounds was detected when the
frequency of the quartz crystal decreased.
6
6
RESULTS AND DISCUSSION
Inhibition of Corrosion
Aromatic compounds: Figure 2 shows the corrosion rate of the carbon steel coupon after being
immersed in LVT (Base Line) and LVT/tetrahydronaphthalene for 20 minutes. Even with 40%
tetrahydronaphthalene, the corrosion rate does not change. Aromatics can adsorb onto a metal
surface15, but the molecule-surface interactions created are insufficiently strong to replace the
water molecules adsorbed onto the iron surface, and therefore do not significantly decrease the
corrosion rate.
Cor. Rate / (mm/year)
1.0
Base Line
0.5
40 % Tetrahydronaphthalene
0.0
0
5
10
15
20
25
Time / min
Figure 2: Corrosion measurements by LPR for aromatics added to the
model oil (Base Line: model oil no Tetrahydronaphthalene) .
Oxygen containing compounds: Small molecule organic acids such as acetic acid [CH3CO2H] are
insoluble as a monomer in oil. Consequently, it was not possible to add acetic acid to the oil
phase and record the corrosion rate. However, glass cell experiments were performed in aqueous
conditions. The acetic acid corrosion rate was recorded by LPR experiments. The conditions
were identical to those outlined for corrosion measurements in a previous section (1 wt. % NaCl,
1 bar CO2, pH 5.0). The corrosion rate was recorded for 10 ppm, 100 ppm and 1000 ppm acetic
acid and the results are shown in Table 3.
Table 3: Dependency of carbon steel’s corrosion rate function of acetic acid
concentration in aqueous solution.
Concentration of
0
10
100
1000
acetic acid (ppm)
Corrosion rate
0.79
0.84
0.96
1.47
(mm/year)
7
7
Corrosion rate increases with acetic acid concentration. However, as shown in Figure 3, long
chain organic acids such as myristic acid [CH3(CH2)12COOH] and naphthenic acids decrease the
corrosion rate.
Cor. Rate / (mm/year)
1.0
Base Line
Naphthenic acid 0.1%
0.5
Naphthenic acid 1%
Naphthenic acid 10%
Myristic Acid 0.1%
Myristic Acid 1%
0.0
0
5
10
15
20
25
Time / min
Figure 3: Corrosion measurements by LPR for oxygen containing compounds added to
the model oil phase.
Long chain organic acids are able to significantly decrease the corrosion rate. Only 0.1%
concentration of myristic acid gives a 48% inhibition of corrosion, while a 1% concentration gives
88% of inhibition of corrosion. Naphthenic acids present the same behavior. These results were
expected, the protection against corrosion by organic acid is well known21,22. Moreover, EIS
experiments showed that organic acids have negligible effect on the corrosion mechanism and
polarization sweeps showed that only the cathodic reaction is inhibited by organic acid while the
anodic reaction stays unchanged.
Sulfur containing compounds: Figure 4 shows the results obtained with sulfur containing
compounds. Dibenzothiophene and dioctyl-sulfide have no significant effect on the corrosion rate.
However, with the addition of 1-tetradecanethiol (a mercaptan) the corrosion rate decreases
significantly. Only 0.1% of 1-tetradecanethiol produces a 44% inhibition of corrosion and 1% of 1tetradecanethiol gives 84% of inhibition of corrosion. This is in good agreement with the
mercaptan literature25. As for the organic acids, EIS measurements show that the corrosion
mechanism is unchanged. However, the polarization sweep shows that 1-tetradecanethiol
induces a strong inhibition of both the anodic and cathodic processes.
8
8
Cor. Rate / (mm/year)
1.0
Base Line
Dibenzothiophene 0.1%
Dibenzothiophene 1%
0.5
Dioctyl sulfide 0.1%
Dioctyl sulfide 1%
1-Tetradecanethiol 0.1%
1-Tetradecanethiol 1%
0.0
0
5
10
15
20
25
Time / min
Figure 4: Corrosion measurements by LPR for sulfur containing compounds added to the
model oil phase.
Nitrogen containing compounds: Figure 5 depicts results obtained with nitrogen containing
compounds. While carbazole cannot adsorb onto iron surface and inhibit corrosion, acridine is
able to adsorb at the metal surface and strongly inhibit corrosion. Only 0.01% of acridine added to
the oil phase produces 83% inhibition of corrosion, while 0.1% acridine present in the oil phase
induces a 100% corrosion inhibition. During EIS experiments polarization resistance as high as
15000 were recorded. It should be noted that the 100% corrosion inhibition associated with
acridine is too high to be representative of all “pyridinic type” compounds found in crude oil.
Therefore, three nitrogen containing compounds have been identified for further study:
benzo[h]quinoline, phenanthridine and 1,10-phenanthroline.
9
9
Cor. Rate / (mm/year)
1.0
Base Line
Acridine 0.01%
0.5
Acridine 0.1%
Carbazole 0.01%
Carbazole 0.1%
0.0
0
5
10
15
20
25
Time / min
Figure 5: Corrosion measurements by LPR for nitrogen containing compounds added to
the model oil phase.
In summary, four classes of compounds have been tested but only three were able to reduce the
corrosion rate (oxygen, sulfur and nitrogen containing compounds). Aromatics were unable to
have a significant effect on corrosion even in very large concentration (40%). Figure 6 shows that
within the same class of compounds not every subclass was able to inhibit corrosion.
Oxygen containing compounds: Small molecules organic acids increase the corrosion rate while
large molecules organic acids have inhibitive properties.
Sulfur containing compounds: Only mercaptans are able to decrease corrosion rates while dialkyl
sulfides and thiophenes have no significant effect.
Nitrogen containing compounds: Pyridinic compounds (basic nitrogen) can adsorb at the metal
surface, this is proved by acridine strong inhibition of corrosion. However, pyrrolic compounds
(neutral nitrogen) such as carbazole have no effect on corrosion.
10
10
Inhibition of Corrosion
100%
75%
50%
25%
Te
t
ra
hy
dr
Ba
on
se
ap
Li
ht
ne
ha
N
le
ap
ne
ht
he
40
ni
%
c
ac
N
id
ap
0.
ht
1%
he
ni
N
c
ap
ac
ht
id
he
1%
ni
c
ac
id
M
10
ys
%
tri
c
Ac
0.
M
1%
D
ys
ib
tri
en
c
zo
Ac
th
1%
io
ph
D
en
ib
en
e
zo
0.
1%
th
io
p
D
he
io
ne
ct
yl
1%
su
lfi
de
D
io
0.
ct
1%
1yl
Te
su
tra
lfi
d
de
e
ca
1%
ne
1t
Te
hi
ol
tra
0.
de
1%
ca
ne
th
io
Ac
l1
rid
%
in
e
0.
01
Ac
%
rid
in
e
C
0.
ar
1
ba
%
zo
le
0.
C
01
ar
%
ba
zo
le
0.
1%
0%
Figure 6: Summary of the inhibitive effect of the compounds tested in this study.
It should be noted that myristic acid and 1-tetradecanethiol have a similar chemical structure
(Table 1). The comparison of these chemicals inhibitive properties suggests that once a chemical
is adsorbed at the metal surface its functionality (oxygen or sulfur) may not be important and that
the length of the carbon chain has the strongest effect on corrosion inhibition. This is a stearic
effect.
Steel Wettability
Base line experiment (model oil): When a spherical water droplet touches a flat steel surface the
water-steel contact angle is 180°. Interfacial forces act and the water droplet spreads across the
steel surface displacing the oil. Therefore the contact angle decreases as a function of time.
Figure 7 shows the evolution of a water droplet in model oil during the first 5 minutes. The final
water-steel contact angle is 58°.
11
11
t=0s
t=1s
t=2s
t=10s
t=20s
t=30s
t=60s
t=120s
t=180s
t=240s
t=300s
t=600s
Figure 7: Evolution of a water droplet in model oil during the first 5 minutes
as the contact angle changes from 180° to 58°.
The same experiment was repeated with surface active compounds added to the oil phase.
Heuristically, the chemicals that were able to have an effect on corrosion are supposed to have a
similar effect on the steel wettability. Figure 8 shows the water-steel contact angle after 5 minutes
and Figure 9 the water-steel contact angle after 2 hours for the chemicals used in this study.
12
12
180°
STEEL HYDROPHOBIC
Contact Angle
135°
90°
45°
STEEL HYDROPHILIC
Ba
se
Te
tra
Li
hy
ne
dr
(m
on
od
ap
el
ht
oi
ha
l)
Ac
le
ne
et
ic
4
0%
Ac
Ac
id
et
10
ic
pp
Ac
Ac
m
id
et
10
ic
0
Ac
pp
id
m
10
00
M
ys
pp
t ri
m
c
Ac
0.
M
1%
N
ys
ap
tri
ht
c
Ac
he
ni
1%
c
N
ac
ap
id
h
0
t
D
he
.1
ib
ni
%
en
c
zo
ac
th
i
d
io
1%
D
ph
ib
en
en
e
zo
0.
th
1%
io
D
ph
io
en
ct
yl
e
su
1%
lfi
D
de
io
1ct
0.
yl
Te
1%
su
tra
l
f
de
id
e
ca
11%
ne
Te
th
t ra
io
l0
de
.1
ca
%
ne
th
C
io
ar
l1
ba
%
zo
le
C
0.
ar
01
ba
%
zo
le
Ac
0
.
rid
1%
in
e
0.
01
Ac
rid
%
in
e
0.
1
%
0°
Figure 8: Water-Steel contact angle after 5 minutes. Organic acids have the strongest
effect on steel wettability.
180°
Contact Angle
STEEL HYDROPHOBIC
135°
90°
45°
STEEL HYDROPHILIC
Te
t
ra
hy
dr
Ba
se
Li
ne
(m
on
od
ap
el
ht
oi
ha
l)
Ac
le
ne
et
ic
4
0%
Ac
Ac
id
et
10
ic
pp
Ac
Ac
m
id
et
10
ic
0
Ac
pp
id
m
10
00
M
ys
pp
tri
m
c
Ac
0.
M
1%
N
ys
ap
tri
ht
c
Ac
he
ni
1%
c
N
ac
ap
id
ht
0
D
he
.1
ib
ni
%
en
c
zo
ac
th
i
d
io
1%
D
ph
ib
en
en
e
zo
0.
th
1%
io
D
ph
io
en
ct
yl
e
su
1%
lfi
de
D
io
ct
10.
yl
Te
1%
su
tra
l
f
de
id
e
ca
11%
ne
Te
th
tra
io
l0
de
.1
ca
%
ne
th
C
io
ar
l1
ba
%
zo
le
C
0.
ar
01
ba
%
zo
le
Ac
0
.
ri d
1%
in
e
0.
01
Ac
rid
%
in
e
0.
1
%
0°
Figure 9: Water-Steel contact angle after 120 minutes. Organic acids effect on steel
wettability is not dependant on time.
Aromatics: the contact angle water-steel remained at 180° (steel hydrophobic) for more than 30
seconds then started to decrease. It reached 130° after 5 minutes and then a stable value of
about 60° in less than 2 hours (the steel surface becoming hydrophilic under 90°). Even if the
final contact angle is the same as the one obtained with model oil, the time needed to reach the
final value is much longer. In this experiment, it seems that the aromatics are adsorbed onto the
13
13
metal surface and the water phase slowly displaces the adsorbed oil phase. This explains why no
effects related to aromatics were found in the corrosion measurements.
The oil layer is displaced more rapidly when the oil is free of aromatics. Even if the final value of
the contact angle is the same, the effect of aromatics must not be neglected, particularly in
intermittent oil-water two-phase flow. When a water droplet touches the pipe wall, this droplet
either wets the pipe or rebounds from it. Without compounds added to the model oil the water
immediately wets steel, while when aromatics are added to the oil phase water droplets can
bounce on surface without wetting it. This effect can strongly limit the corrosiveness of dispersed
oil-water flow.
Oxygen containing compounds: small molecule organic acids do not have an effect on steel
wettability. However, the strongest effect is found with the high molecular weight organic acids.
While steel is naturally hydrophilic, once it is wet with model oil and myristic acid or naphthenic
acids, steel becomes completely hydrophobic. Even after a long period of time, the water droplet
touching the steel surface never wetted it. The contact angle never went below 180°.
The effect of naphthenic acids on wettability is even stronger than the effect of aromatics. It is
possible to conclude that if a straight pipe is pre-wetted with oil and naphthenic acids, dispersed
water droplets in the oil flow are not going to be corrosive.
Sulfur containing compounds: the effect on wettability is weaker than for the organic acids.
Dibenzothiophene and dioctyl sulfide do not change the steel wettability. Even tetradecanethiol,
which is a good corrosion inhibitor, is only able to change the contact angle from 58° to 95°. It is a
significant change in the wettability of the steel. However, this change of contact angle is not
sufficient to affect the wettability and reduce corrosion significantly by itself. Once a water droplet
wets the pipe, corrosion happens irrespectively from the contact water-steel angle.
Nitrogen containing compounds: were tested using model oil and 40% tetrahydronaphthalene. No
significant effect on the steel wettability is found for the nitrogen containing compounds. It is
possible that the strong effect of the aromaticity of the oil is masking the effect of the nitrogen
containing compounds
Effect of the Flow on Corrosion
Oil-water interfacial tension measurements were conducted and the results are shown in Figure
10. None of the chemical tested are able to significantly decrease the interfacial oil-water tension
except for the long chain organic acids (myristic acid and naphthenic acids). Organic acids are
able to change the interfacial tension, 1% naphthenic acid reduces the interfacial oil-water tension
by half.
14
14
Interfacial Tension OilWater / (dyne/cm)
50
40
30
20
10
Te
tra
Ba
se
Li
hy
ne
dr
(m
on
od
ap
el
ht
oi
ha
l)
Ac
le
ne
et
ic
4
0%
Ac
Ac
id
et
10
ic
pp
Ac
Ac
m
id
et
10
ic
0
Ac
pp
id
m
10
00
M
ys
pp
tri
m
c
Ac
0.
M
1%
ys
N
ap
t ri
c
ht
Ac
he
ni
1%
c
N
ac
ap
i
d
ht
0.
D
he
1%
ib
ni
en
c
zo
ac
th
id
io
1%
D
ph
ib
en
en
e
zo
0.
th
1%
io
D
ph
io
en
ct
yl
e
su
1%
lfi
de
D
io
ct
10.
yl
Te
1%
su
tra
l
f
de
id
e
ca
11%
ne
Te
th
tra
io
l0
de
.1
ca
%
ne
th
C
io
ar
l1
ba
%
zo
le
C
0
.0
ar
1%
ba
zo
le
Ac
0.
rid
1%
in
e
0.
01
Ac
rid
%
in
e
0.
1
%
0
Figure 10: Water-oil interfacial tension function of the chemicals
added to the oil phase (acetic acid is added to the water phase).
Figure 11, shows the effect of naphthenic acids on the transition line water-wetting to oil-wetting
calculated using Equation 2. Such a major change in the interfacial tension has only a small effect
on the flow. A decrease in the oil-water interfacial tension is beneficial from a corrosion point of
view (water is more easily dispersed in the oil flow). However, to have a significant change in the
flow pattern, the oil-water interfacial tension reduction has to be much greater that 50%.
25%
Water Cut
20%
15%
OIL WETTING
WATTER WETTING
10%
Model oil
5%
Model oil + 0.1% naphthenic acid
Model oil + 1% naphthenic acid
0%
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
-1
Velocity / m.s
Figure 11: Phase wetting map for model oil (with naphthenic acids) calculated
using the water wetting model.
15
15
Adsorption Measurement
The results of the adsorption measured with a QCM are shown in the Figure 12. The mass
adsorbed has been converted to a film thickness assuming that the film created during the
adsorption is homogenous.
Aromatics were not tested since the high concentration of tetrahydronaphthalene changes the
viscosity of the model oil. A change in the viscosity implies a change in the quartz crystal’s
frequency much larger than the effect of the mass adsorbed.
For oxygen containing compounds acetic acid was not tested. The results for myristic acid and
naphthenic acids are shown in Figure 12. The adsorption of myristic acid and naphthenic acids
are similar, a rapid adsorption with a maximum about 1g·cm-2. This quick response suggests
that the adsorption of organic acids is physisorption: i.e., rapid adsorption through weak
intermolecular interactions (Van der Waals forces).
The adsorption of sulfur containing compounds gave two different responses. Dibenzothiophene
and dioctylsulfide have a rapid adsorption rate with a maximum about 0.5g·cm-2. The 1tetradecanethiol adsorption was slow but steady. The maximum mass adsorbed measured was
0.9g·cm-2. Such slow adsorption is likely to be chemisorption.
Nitrogen containing compounds were not tested for the same reason as stated for aromatics.
8
Naphthenic acid 1%
1.0
Myristic Acid 1%
6
1-Tetradecanethiol 1%
0.8
0.6
4
0.4
Octylsulfide 1%
2
Dibenzothiophene 1%
Film Thickness / nm
Mass Adsorbed / (g/cm2)
1.2
0.2
0.0
0
0
5
10
15
20
25
30
35
40
Time / min
Figure 12: Evolution of the mass adsorbed for different
surface active compounds tested as a function of time
Using the corrosion measurement (Figure 6) and the results of the QCM (Figure 12), a graph that
correlates inhibition and adsorption can be generated. Each chemical tested produces one data
point and the results are shown in Figure 13. The results indicate a linear relationship between
inhibition of corrosion and the mass adsorbed at the metal surface. However, testing of only 5
chemicals is insufficient for the development of a universal conclusion.
16
16
100%
Inhibition of Corrosion
Myristic Acid 1%
1-Tetradecanethiol 1%
80%
Naphtenic acids1%
60%
40%
Dibenzothiophene 1%
Octylsulfide 1%
20%
0%
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Mass Adsorbed / (g/cm )
2
Figure 13: Relationship between mass adsorbed and inhibition of corrosion
CONCLUSION
This study was conducted to test three hypotheses related to corrosion inhibition and alteration of
phase wetting of a mild steel surface by compounds found in crude oil:
1 - Corrosion inhibition is induced by the accumulation of surface active compounds at
the metal surface.
2 - Wettability of steel is altered by accumulation of surface active compounds at the
metal surface.
3 - Flow pattern is modified by accumulation of surface active compounds at the oil-water
interface.
All three hypotheses are confirmed. Surface active compounds found in the crude oil adsorb onto
the metal surface as the QCM measurements unambiguously demonstrated. The effect of these
compounds on corrosion has been quantified, the effect on steel wettability has been measured
and change of the oil-water interfacial tension has been recorded. However, as expected, not all
surface active compounds produced the same effect on corrosion, wettability and change of the
flow pattern.
17
17
NOTATION
D
f
g
u




m
o
s
w
Internal diameter of the pipe
Friction factor
Gravity constant
Velocity
Pipe inclination
Percentage of fluid
Density
Interfacial tension oil-water
Mixture oil-water
Oil
Steel
Water
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